Remaining problems ingeodesy/astrometry VLBI andapproaches to their solutions

Leonid Petrov NASA GSFC

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I. Science with astrometry/geodesy VLBI

1. VLBI/Gaia offsetsVLBI Radio Fundamental Catalogue (15,740 sources) on 2019.03.08 andGaia DR2 (1.69 · 109 objects)

Green: 9,453 VLBI/Gaia matches P < 0.0002

Blue: VLBI sources without Gaia matchesSlide 2(22)

The histograms of the distribution of the position angle of Gaia offset with respect to VLBIposition counted from the jet direction counter-clockwise.

Top left (a):Top right (b):Bottom left (c):Bottom right (d):

all the matches with known jet directions;the matches with σψ < 0.3 rad;the matches with σψ < 0.3 rad and arc-lengths < 2.5 mas;the matches with σψ < 0.3 rad and arc-lengths > 2.5 mas.

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Measurement of VLBI/Gaia offsets allows

• to identify the area where a flare occurs: at the core, accretion disk, or hotspot

• to measure a size of the optical jet and its relative flux density

• to measure a share of synchrotron emission wrt accretion disk emission

• to measure a jitter in optical positions

Solved problems:

• We have established the nature of ψ = 0 peak

• We have indirectly detected jitter in Gaia positions

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Remaining problems of VLBI/Gaia offset analysis

• What is the nature of ψ = π peak?

• How the size and strength of the optical jet is related to other AGNproperties?

• Quantitative analysis of the size and strength of the optical jet sample

• How to correct Gaia positions for jitter?

What needs to be done?

• Deep images of Gaia counterparts

• Improvement in position accuracy down to 1–1.5 nrad for > 4, 000 sources.

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2. Variable core-shift

What we know:

• When core-shift d ∼ f−1 it has no effect on τiono−free.

• d ∼ f−1 corresponds to equi-partition state and it is common

• d ∼ f−1 is violated during flares has effect on τiono−free.

• core-shift is changed during flares.

• core-shift changes are large.

Plavin et al, (2019) MNRAS485, 1822–1842

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Measurement of core-shift variability allows

• to probe the inner optically thick part of the jet

• to understand the mechanism of AGN flares

Problems:

• How to measure core-shift routinely?

• How to compute the contribution of the core-shift to group delay?

• How to assess the contribution of the core-shift on source positions withoutdirect measurements?

• For how long the state of equi-partition (d ∼ f−1) is violated?

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Technique improvement

1. Computation of the source structure contribution

We have ∼ 80k images of over 14,000 sources. How to use them?

Problems:

• What is the best way to compute structure delay? CLEAN component,direct visibility usage, Gaussian models?

• How to get images from geodetic experiments?– We need care about amplitudes (Tsys, gain curves measurement)– Need develop a new imaging procedure based on perturbation of known

images.

• How to make images routinely?

• How to find an invariant point on the image?

• How to assess an uncertainty of structure delay?

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What needs to be done?

• Change our mind

• Set the goal. Enough demonstrations, let us do production!

• Develop infrastructure for imaging every experiment

• Routine process experiments on the visibility level

• Focus on tuning hardware for providing excellent amplitude calibration

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2. Accounting for the atmosphere turbulence

1. 1st approximation:

• Atmosphere is uniform:τ(t, E,A) = τz(t) ∗m(e) orτ(t, E,A) = τz(t) ∗m(e+ η cos(A) + ε sin(A))

• τ is uncorrelated time and space: Cov(τ1, τ2) = 0

2. 2nd approximation:

• Atmosphere is non uniform: τ(t) 6= f1(e) ∗ f2(A)• τ is correlated in time and space:Cov(τ(t, e1, A1), τ(t, e2, A2)) 6= 0Cov(τ(t1, e, A), τ(t2, e, A)) 6= 0

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Approaches:

• Estimation model: beyond spherical harmonics degree/order 1? Slepianbasis?

• Covariance matrix: how to get it?

– Global regression model

– Station-dependent regression model

– Computation based on the output of numerical weather models

What needs to be done:

• Set the goal. Enough demonstrations, let us do production!

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3. Mitigation of the polarization impurity impact

A. Roy 2010,Ph.D. Thesis, p. 66.

Polarization leakage results in

• contribution to group delay

• phase misclosure

• dependence on parallactic angle

• loss of sensitivity

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Linear polarization makes the impact worse.

• leakage term degrades computation of polarization bandpass

• cross-talk with intrinsic source polarization

What needs to be done:

• Change mind: we do care of polarization now

• Evaluate D-term and monitor it

• Routine process experiments on the visibility level

• Monitor intrinsic source polarization

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4. Mitigation of RFI

Satellite radio

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Internal RFI

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Internal RFI

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What needs to be done?

• Realize that we will have to live with RFIs

• Automatic RFIs detection and cleaning the data

• Analyzing the RFI source and build an empirical RFI model (t,e,A,f)

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5. Scheduling

VGOS hardware provides us more freedom to make better schedules.

Are we ready to use this freedom?

Remaining problems:

• The problem to generate the best schedule is not solved theoretically

• N-step lookahead versus full schedule optimization?

• To optimize the schedule for optimal accuracies versusformal uncertainties

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6. Automation

Why do we still do VLBI data analysis manually??

Goal: reach full automation:

• workflow

• flagging, masking, catching breaks at the visibility level

• flagging, masking, catching breaks at the group delay level

• flagging, masking, boxing image components

An Analyst should use brain for analyzing logs, not hands for moving a mouse.

Feasible? YES! Key Stone Project (1994–1996)

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III. Measurement of antenna gravity deformations

S. Bergstrand, et al., J. Geodesy, in press 2019.

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Unaccounted antenna gravity deformations result in position biases, mainlyalong Up.

So far, only 6 antennas were measured

Problems:

• How to organize measurement of all antennas?

• How to account for events that may results in change of gravitydeformations?

• How to “measure” decommissioned antennas?

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Summary:

• Scientific problems: VLBI/Gaia offset and core-shift.

• Data analysis: atmosphere turbulence, source structure contribution,accounting for polarization impurity, RFI mitigation.

• Logistics: make VLBI fully automatic; new scheduling.

• Measurement: antenna gravity deformation.

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Difficulties in dealing with source structure:

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Where is the core, where is the jet?

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